✨ feat(server): add go_version to /api/info response

Adds a "go_version" field to InfoResponse, populated via runtime.Version().
Useful for ops debugging (which Go version is running in production?).
BDD scenario @critical asserts the field is non-empty. Doc + ADR-0026
updated.

pkg/server/server.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"net/http"
 	"os/signal"
+	"runtime"
 	"syscall"
 	"time"
 
@@ -453,6 +454,7 @@ type InfoResponse struct {
 	UptimeSeconds int64  `json:"uptime_seconds"`
 	CacheEnabled  bool   `json:"cache_enabled"`
 	HealthzStatus string `json:"healthz_status"`
+	GoVersion     string `json:"go_version"`
 }
 
 // handleHealthz godoc
@@ -500,6 +502,7 @@ func (s *Server) handleInfo(w http.ResponseWriter, r *http.Request) {
 		UptimeSeconds: int64(time.Since(s.startedAt).Seconds()),
 		CacheEnabled:  s.cacheService != nil,
 		HealthzStatus: "healthy",
+		GoVersion:     runtime.Version(),
 	}
 
 	// Cache key